Suppression of unstable burning using finely divided metal oxides

ABSTRACT

1. A SOLID PROPELLANT COMPOSITION WHICH COMPRISES A CURED INTIMATE MIXTURE OF A NONMETALLIC OXIDIZING SALT, A RESIN BINDER SELECTED FROM THE GROUP CONSISTING OF POLYURETHANE RESIN, POLYESTER RESIN, ACRYLATE RESIN, POLYSULFIDE POLYMER, NITROCELLULOSE PLASTICIZER, AND NITROPOLYURETHANE RESIN, AND AS A RESONANCE SUPPRESSOR, A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE, ZIRCONIUM OXIDE, SILICON DIOXIDE, THORIUM OXIDE, TITANIUM OXIDE LANTHANUM OXIDE, AND MIXTURES THEREOF, IN AN AMOUNT EFFECTIVE TO SUPPRESS RESONANT BURNING UP TO AN AMOUNT NOT GREATER THAN THAT EQUIVALENT TO ABOUT 5 PERCENT BY WEIGHT OF THE PROPELLANT COMPOSITION, SAID RESIN BINDER BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 5 PERCENT AND ABOUT 55 PERCENT BY WEIGHT AND THE SOLID NON-METALLIC OXIDIZING SALT BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 95 AND ABOUT 45 PERCENT BY WEIGHT, ALL PERCENTAGES BEING GIVEN BY TOTAL PROPELLANT WEIGHT BASIS.

y 2, 1974 R. w. LAWRENCE ETAL 3,822,154

SUPPRESSION OF UNSTABLE BURNING USING FINELY DIVIDED METAL OXIDES 2 Sheets-Sheet 1 Filed Oct. 1 1962 CONTROL 0 O O mmm w l 0 TIME (SECONDS) O 0 O m 0 m m a 4 TIME (SECONDS) y 2, 1974 R. w. LAWRENCE EFAL 3,822,154

SUPPRESSION OF UNSTABLE BURNING USING FINELY DIVIDED METAL OXIDES 2 SheetsSheet 2 Filed 001;.

0 Z 2m IID N F m mE a 6 M T 0 0 2 o 1 O 0 O o O 0 o O o 0 m m m e 4 65% wmzwwmmm mmmE I0 LO TIME (SECONDS) O 0 w m 0 O 6 Q 8 4 65m: mmDmmmma EmmSZIQ United" States Patent C 1 fice This invention relates to novel solid propellant compositions and in particular to novel propellant compositrons comprising a polymeric resin binder, a relatively small quantity of an oxide of aluminum, zirconium, silicon, thorium, titanium or lanthanum, and a finely divided oxidizing agent.

Solid propellant compositions are ordinarily composed of a resin fuel and an oxidizing material, the oxidizing material being intimately dispersed in the fuel. The ignition and burning properties of a propellant composition as well as its physical properties are dependent to a large extent upon the particular resins employed as fuels. In the novel propellant compositions of this invention, the preferred fuel is a cross-linked polyurethane which yields propellants of unexpectedly superior physical properties and performance characteristics.

We have discovered that the presence of a very small quantity of an oxide of aluminum, zirconium, silicon, thorium, titanium or lanthanum in a propellant grain effects a substantial and unexpected improvement in its burning characteristics. This discovery forms the essence of our invention and hence the invention is not limited to propellants containing the preferred polyurethane binders but is broad enough to encompass the field of solid propellants in general. Thus, propellants containing wellknown binders such as, for example, nitropolyurethane, polyesteracrylate, rubber (butyl, polysulfide), etc., are all within the scope of our invention.

We have found that the presence of a small quantity of an oxide of aluminum, zirconium, silicon, thorium, titanium or lanthanum in a polwrethane propellant grain suppresses reasonant burning of the propellant when in use. Resonant burning, or as it is sometimes called, unstable burning, is a phenomenon encountered in the firing of rocket motors in which high frequency pressure oscillations disturb the normal burning process and in severe cases even rupture the combustion chamber.

Methods heretofore employed for controlling reasonant burning have involved the use of mechanical dampers such as resonance rods or baflies or the incorporation of an additive such as potassium perchlorate into the propellant grain. However, there are disadvantages in the use of these methods. For example, the mechanical dampers add dead weight to the unit and the use of an additive such as potassium perchlorate results in the production of smoke which is undesirable in many instances.

In contrast to the above-described methods of suppressing resonant propellant burning, the use of small quantities of aluminum, zirconium, silicon, thorium, titanium or lanthanum oxides, or a mixture of several or all of these, adds substantially no dead weight and does not significantly increase the temperature coefiicient of chamber pressure. Also, polyurethane propellants incorporating the above oxides burn with less smoke than grains incorporating potassium perchlorate as a resonance suppressor.

The novel polyurethane propellants of our invention can be cured at low temperatures and in addition exhibit no measurable heat of reaction. As a result of these unique properties they are not subject to shrinkage and hence have substantially no internal strains in the cured state. Composite propellant systems other than polyurethane 3,822,154 Patented July 2, 1974 propellant systems have all been severely restricted in their use because of high heats of reaction and the need for high cure temperatures which produce shrinkage and internal stresses. These faults have imposed severe restrictions upon the size of solid propellant motors because of their tendency to crack as a result of internal stresses. The preferred propellants of this invention are not subject to such size limitations because of the use of crosslinked polyurethanes as the resin fuel component.

In addition to their high specific impulse and their freedom from cracking, the polyurethane propellants have tenacious adhesive properties which are suflicient to bond them directly to rocket chamber linings. This permits optimum utilization of the available space in the rocket motor and simplifies manufacturing techniques. Our polyurethane propellants are also possessed of many other desirable properties such as rubbery mechanical qualities, low brittle point, excellent resilience, and superior aging properties.

Our novel solid propellants can be used as the primary propulsion source in rocket-propelled vehicles or as a propellant for artillery missiles. When used as the primary propulsion source for rocket vehicles, they can be conviently ignited by a conventional igniter, as for example, the igniter disclosed in assignees U.S. Pat. No. 3,000,312, issued Sept. 19, 1961. The propellant is preferably cast directly in the rocket chamber in which it is to be fired and is restricted on one or both ends in a conventional manner with a relatively slow burning inert resin, such as polyurethane or a polyester resin. The restriction is preferably accomplished by applying a relatively thin coating of the inert resin to the inner surfaces of the rocket chamber lining prior to casting the propellant therein. Rocket chambers such as those in which our novel solid propellants are employed are ordinarily of a conventional type having one open end which leads to a venturi rocket nozzle. Upon ignition, large quantities of gases are produced and exhausted through the nozzle to create propulsive force.

The polyurethane binders used in our propellants are prepared by reacting a compound having two or more active hydrogen containing groups capable of polymerizing with an isocyanate, with an organic compound having as the sole reacting groups, two or more isocyanate or isothiocyanate groups. The compounds having the active hydrogen containing groups is preferably an organic compound having as its sole reacting groups, hydroxyl or thiol groups.

It will be apparent that, where there are more than two active hydrogen, isocyanate, or isothiocyanate groups present in any of the polyurethane reactants, the molecular structure of the resulting polyurethane binder will be at least to a certain extent of a cross-linked nature. The cross-linking is accomplished when all three functional groups of a sufiicient number of the trifunctional molecules undergo the urethane reaction with other groups present in the mixture thus resulting in a product having a three-dimensional molecular structure rather than mere aggregates of linear chains as is the case when bifunctional reactants are employed.

Where bifunctional reactants, such as dihydroxy compounds and diisocyanates, are employed to produce the polyurethane binders for our novel propellants, it is necessary to also employ a cross-linking agent to assure a product having the cross-linked structure essential to this invention. Cross-linking agents can also be used with polyurethane reactants having more than two functional groups, such as triols and/or triisocyanates, within the scope of this invention. Compounds suitable as crosslinking agents for our polyurethane binders are those organic compounds having as the sole reacting groups three or more groups polymerizable with active hydrogen containing groups or isocyanate groups.

It will be appreciated that in any given batch of propellant the individual polyurethane molecules may vary in number of repeating units for several to tens of thousands of these units. Hence, molecular weight figures on polyurethanes represent statistical averages. The exact nature of terminal groupings is not known and will vary depending upon whether plasticizers, polymerization catalysts, etc., are present. Moreover, a given molecule may even form a ring and thus leave no dangling radicals.

It is evident from the above that a wide variety of polyurethane binders for the propellants of this invention can be prepared by varying the starting materials.

The isocyanate starting materials for our polyurethane binders are preferably diisocyanates but not necessarily so since, as explained above, other polyisocyanates (such as triisocyanates) or polyisothiocyanates may be employed within the scope of the invention if desired.

Our preferred diisocyanate compounds can be saturated or unsaturated; aliphatic or aromatic; open or closed chain, and, if the latter, monocyclic or polycyclic; and substituted or not by groups substantially unreactive with isocyanate or hydroxyl groups such as, for example, ketone, halogen, ester, sulfide, or ether groups. The following diisocyanate compounds are particularly suitable as reactants for the preparation of binders for our novel polyurethane propellants:

(a) Alkane diisocyanates, such as:

ethylene diisocyanate;

trimethylene diisocyanate; propylene-1,2-diisocyanate; tetramethylene diisocyanate; butylene-1,3-diisocyanate; decamethylene diisocyanate; octadecamethylene diisocyanate; etc.

(b) Alkene diisocyanates, such as:

1-propylene-1,2-diisocyanate; 2-propylene-1,2-diisocyanate; l-butylene-1,2-diisocyanate; 3-butylene-1,2-diisocyanate; l-butylene-1,3-diisocyanate; 1-butylene-2,3-diisocyanate; etc.

(c) Alkylidene diisocyanates, such as:

ethylidene diisocyanate; propylidene-l,l-diisocyanate; propylidene-2,2-diisocyanate; etc.

(d) Cycloalkylene diisocyanates, such as:

cyclopentylene-1,3-diisocyanate; cyclohexylene-1,3 diisocyanate; cyclohexylene-1,2-diisocyanate; cyclohexylene-1,4-diisocyanate; etc.

(e) Cycloalkylidene diisocyanates, such as:

cyclopentylidene diisocyanate; cyclohexylidene diisocyanate; etc.

(f) Aromatic diisocyanates, such as:

m-phenylene diisocyanate; a-phenylene diisocyanate; p-phenylene diisocyanate; 1-methyl-2,4-phenylene diisocyanate; naphthylene-1,4-diisocyanate; diphenylene-4,4-diisocyanate; 2,4-tolylene diisocyanate;

durene diisocyanate;

2,6-tolylene diisocyanate; 4,4'-diphenylmethane diisocyanate; 1,5-naphthalene diisocyanate; methylene-bis (4-phenylisocyanate) 2,2-propylene-bis- (4-phenylisocyante) etc.

The preferred hydroxy starting materials for our polyurethane binders are dihydroxy compounds having the general formula: HO--R-OH; where R is a divalent organic radical. The hydroxy groups on the above compounds can be of any type suitable for the urethane reaction with isocyanate groups such as, for example, alcohol or phenolic hydrox groups. The following dihydroxy compounds are particularly suitable as reactants for the polyurethane binders of this invention:

(1) Alkane diols having a chain length of from 2 to 20 carbon atoms inclusive, such as:

2,2-dimethyl-1,3-propanediol; ethylene glycol; tetramethylene glycol; hexamethylene glycol; octamethylene glycol; decamethylene glycol; etc.

(2) Alkene diols, such as:

l-propylenel ,2-diol; 2-propylene- 1,3-diol; 1-butylene-1,2-diol; 3-butylene-1,2-diol; l-hexylene1,3-diol; 1-butylene-2,5-diol; etc

(3) Cycloalkylene diols, such as:

cyclopentylenel ,3-diol; cyclohexylene-1,2-diol; cyclohexylenel ,3-diol; cyclohexylene-l,4-diol; etc.

(4) Aromatic diols, such as:

catechol;

resorcinol;

quinol;

1-methyl-2,4-benzenediol;

2-methyll ,3-naphthalenediol; 2,4-toluenediol;

xylylene-1,3-diol;

1,5 -naphthalenedimethanol; 2-ethyl-1-phenyl-3-butene-1,2-diol; 2,2-di(4-hydroxyphenyl)propane; etc.

(5 Aliphatic ether diols and amido diols, such as: di(p-hydroxyethyl) ether;

etc.

Other dihydroxy compounds suitable for the polyurethane reaction of this invention are polyesters such as those obtained from the reaction of a dihydric alcohol such as ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, or hexamethylene glycol with a dicarboxylic acid such as succinic acid, adipic acid, sebacic acid, oxadibutyric acid, sulfodipropionic acid, and related compounds. The polyesters most suitable for purposes of this invention are those having a molecular weight from about 1000 to about 2500. In preparing polyesters such as these, the dihydric component is permitted to react with the dicarboxylic acid component to produce the polyester. Mixtures of polyesters and an olefin such as styrene, vinyl acetate, or the like, are particularly suitable for purposes of this invention. The olefin does not react with any of the hydroxy groups present in the mixture, nor does it interfere in any way with the subsequent reaction between these hydroxyl groups and the isocyanate groups in the polyurethane reaction mixture. Neither does it interfere with any reactions of cross-linking agents present in the mixture. The principal function of the olefin is to permit linkage of the polyester molecules together through addi-- tion polymerization.

The above-mentioned polyesters can be prepared from either saturated or unsaturated dihydric alcohols and saturated or unsaturated dicarboxylic acids. The anhydrides of any of the dicarboxylic acids can be substituted for all or part of any of them in the preparation of polyesters suitable for the polyurethane reaction of our invention. The usual and preferred manner of making suitable polyesters is to react a mixture of an unsaturated dicarboxylic acid (such as adipic acid, sebasic acid, or the like) or anhydride and a saturated or aromatic dicarboxylic acid or anhydride with a dihydric alcohol. Examples of unsaturated dicarboxylic acids which can be employed are: maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, etc.

In addition to the polyesters, polyethers such as polyethylene ether glycols, polypropylene ether glycols, other polyalkylene ether glycols, and mixtures or copolymers thereof having molecular Weights of from about 400 to about 10,000 can be utilized as dihydroxy reactants of the polyurethane reaction of this invention.

Polysulfides having two or more thio groups, such as ethylene disulfide and the Thiokols produced by Thiokol Corporation, and polysulfides with glycol end groups such as those having the general formula,

where x is a whole number, are other suitable reactants for the polyurethane reaction of our invention.

It will be appreciated by those skilled in the art that mixtures of suitable polyhydroxy and/or polyisocyanate compounds can be used for purposes of this invention if desired.

It is well known to those skilled in the art that polyisothiocyanates and polythiol compounds react to yield urethane-type products in the same manner as polyisocyanate and polyol compounds. Consequently, polyisothiocyanates and polythiols corresponding to the polyisocyanates or polyhydroxy compounds taught herein can be employed for the preparation of propellant binders useful in this invention. For example, diisothiocyanates such as butylene-1,3-diisothicyanate; ethylidene diisothiocyanate; cyclohexylene-1,2-diisothiocyanate; cyclohexylidene diisothiocyanate; p-phenylene diisothiocyanate; and xylylene- 1,4-diisothiocyanate; etc., react with dithio compounds such as decamethylene dithiol; thioresorcinol; ethylene bis-(thioglycolate); etc., to yield polythiourethane compounds which are suitable as binders for our novel propellant compositions. Any mixture of the diisocyanates and/or diisothiocyanates suitable as reactants for the propellant binders of this invention can be reacted with any mixture of diols and/or dithios disclosed as suitable for the purpose within the scope of our invention.

It will be appreciated by those skilled in the art that a great variety and number of polyfunctional organic compounds Will serve as cross-linking agents for the polyurethane binders of this invention. As indicated above, any organic compound having as its sole reacting groups three or more groups polymerizable with hydroxy or isocyanate groups is a suitable cross-linking agent for purposes of this invention. This includes not only the polyfunctional hydroxy, thiol, isocyanate, and isothiocyanate compounds but also compounds containing other groups polymerizable with either hydroxy or isocyanate groups. For example, compounds with three or more groups containing reactive hydrogen which are capable of polymerization with isocyanates can be employed as cross-linking agents within the scope of this invention. Examples of compounds of this class are proteins and synthetic polyamides such as polyhexamethylene adipamides. The crosslinking agents of this invention can be saturated or unsaturated; aliphatic or aromatic; open or closed chain and, if the latter, monocyclic or polycyclic; and substituted or not by groups substantially unreactive with isocyanate or hydroxyl groups such as, for example, ketone, halogen, ester, sulfide, or ether groups.

Examples of compounds which we have found to be particularly suitable as cross-linking agents are glycerol monoricinoleate; glycerol triricinoleate (referred to hereinafter as GTRO); 1,2,6-hexanetriol; methylene bis- (orthochloraniline); monohydroxyethyl trihydroxypropyl ethylenediaimne; polyaryll polyisocyanates; pentaerythritolpropylene oxide adduct; N,N,N,N'-tetrakis (2-hydroxypropyl) ethylenediamine; triethanolamine; trimethylolpropane; and triisocyanates, such as toluene-2,4-6-triisocyanate.

Other substances suitable as cross-linking agents are glycerol, sorbitol, dextrin, starch, cellulose, ethyl cellulose, cellulose acetate, polyvinyl acetals, polyvinyl ketals, polyvinyl alcohol, diethylenetriamine, polyvinyl mercaptans and shellac.

As in the case of the polyurethane reactants, mixtures of the various cross-linking agents can be employed with in the scope of this invention.

While polyurethane binders are preferred for purposes of this invention, it is within the scope of the invention to employ any other solid propellant binder in our novel propellants. For example, resinous binders such as rubbers, polysulfides, rubber-polysulfide mixtures, other combustible polymeric organic materials, etc., are all suitable for this purpose. Examples of combustible polymeric organic materials suitable as propellant binders are phenol-aldehyde resins, polyester resins, acrylate resins, and polyalkylene resins.

Examples of rubber binders which can be employed within the scope of our invention are polyisobutylene, butyl rubber, butadiene-styrene copolymers such as Buna-S, a butadiene-acrylonitrile copolymer such as Buna-N, highly polymerized vinyl alcohols in a plasticized state such as polyvinyl alcohol and chloroprene polymers such as Neoprene. The polysulfides suitable as solid propellant binders are exemplified by polyalkylene sulfides such as that resulting from the condensation of ethylene dichloride and sodium tetrasul-fide. A more complete description of rubber and polysulfide propellant binders can be found in assignees US. Pat. No. 3,012,866, issued Dec. 12, 1961.

The so-called polyester resins suitable for use as solid propellant binders are formed by reacting a polyhydric alcohol with a polycarboxylic acid and copolymerizing therewith a monomeric olefinic components such as a vinyl, allyl, or other olefin compatible with the resin. To permit heteropolymerization between the polyester and olefin components, the polyesters are provided with some unsaturation through the incorporation therein of unsaturated polycarboxylic acid or anhydride and/or unsaturated polyhydric alcohol.

Saturated polycarboxylic acids useful in compounding the polyester resins are, for example, the aliphatic dibasic acids, including oxalic, malonic, succinic, glutaric, adipic, pimelic, sebacic, azelaic acids, etc., and the unsaturated carbocyclic acids useful as the acidic components in forming polyester resins are maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid, etc. The anhydrides such as itaconic anhydride and phthalic anhydride may likewise be used for supplying the desired unsaturation.

Regardless of which of the saturated acids are used, the degree of unsaturation necessary to provide cross-linkage with the vinyl, allyl, or other ole-finic components may be obtained by the addition of any of the above-named unsaturated acids or their anhydrides.

The alcohols that can be used are not limited to the dihydric alcohols as other polyhydric alcohols such as the trihydric and higher polyhydric alcohols may be used. These afford additional possibilities for cross-linking and as a consequence the toughness and brittleness of the final resin may be controlled as desired.

For the polyhydric alcohol component any of the following alcohols may be used: dihydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol,

propylene glycol, etc.; a trihydric alcohol such as glycerol; tetrahydric alcohols such as the erythritols, pentaerythritols, etc.; pentitols which include arabitol, adonitol, xylitol, etc.; hexitols including mannitol, sorbitol, dulcitol, etc.; heptitols such as persitol, volamitol, etc.; or mixtures of any of the above alcohols may be also employed if desired.

The olefinic component of the polyester resin binders may be styrene; vinyl acetate, acrylic acid esters; methacrylic acid esters; allyl compounds such as allyl diglycol carbonate, diallyl maleate, and diallyl glycolate; and other unsaturated components such as propylene, butadiene, etc.; as well as derivatives of any of the above substances which are capable of polymerization with the resin. In general, any olefin which will polymerize with the resin to form a solid grain may be employed; this includes all unsubstituted olefins and in addition many substituted olefins.

The polyester resins suitable as propellant binders and their methods of preparation are more fully disclosed in assignees US. Pat. No. 3,031,288, issued Apr. 24, 1962.

Acrylate resin binders within the scope of this invention comprise copolymers of any two or more reduced oxygen-containing polymerizable monomers such as alkenoic acids, alkenoic acid esters, dialkenyl diglycolates, dialkylene diglycol bis-(alkenyl carbonate), alkenyl phthalates, etc. Examples of reduced oxygen-containing polymerizable monomers suitable for acrylate propellant binder formation are the acrylates and methacrylates such as methyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, butyl methacrylate, propyl methacrylate, diethylene glycol bis-(allyl carbonate), glycidol allyl ether, diallyl phthalate, diallyl diglycolate, diallyl maleate, diallyl fumarate, etc.

Other acrylate binders suitable for use in our invention are prepared by copolymerizing polymerizable substances containing unreduced oxygen in the molecule, such as the nitro and nitroether-substituted alkenoic acids and esters. Specific examples of nitro-containing monomers which copolymerize to form acrylate propellant binders are 2-nitroethyl acrylate; the nitrobutyl acrylates; 2,2-dinitropropyl acrylate; 2,2,3,3-tetranitrobutyl acrylate; and 2,2,3,3- tetranitrobutyl methacrylate.

Still other acrylate binders comprise copolymers of any one or more of the above-mentioned reduced oxygencontaining monomers and any one or more of the abovementioned monomers containing unreduced oxygen in the molecule. These binders, as well as those acrylate binders referred to above, and their methods of preparation are more fully described in assignees copending US. Patent Application Ser. No. 321,941, filed Nov. 21, 1952, and now abandoned.

Polyurethane resins containing unreduced oxygen are suitable binders for the propellants of our invention. Such binders can be prepared by condensing nitro-containing isocyanates and nitro-containing alcohols, as more fully disclosed in assignees copending US. Patent Application Ser. No. 728,491, filed Apr. 14, 1958.

In the preparation of the nitro-substituted polyurethanes of application Ser. No. 728,491, both the alcohol and isocyanate starting materials may contain nitro groups. However, this is not necessary and it is sufiicient if the nitro groups are initially present on only an alcohol or isocyanate starting material. The nitro-substituted polyurethanes (hereinafter referred to as nitropolyurethanes) can be cross-linked or not as desired.

Polyurethane propellants can be prepared with any degree of nitro saturation and all are suitable as propellant binders. It is not necessary to employ an oxidizing salt in nitropolyurethane propellant grains having sufficient oxygen present in the nitro groups to permit satisfactory burning of the propellant after combustion has been initiated.

Examples of alcohols useful for the preparation of nitropolyurethane propellant binders are lower alkylene diols such as ethylene glycol, 1,3-propanediol, and 1,2- pentanediol; nitroalkylene diols such as 2-methyl-2-nitro- 1,3-propanediol, 4,4,6,8,8-pentanitro 1,11 undecanediol, 2,2,4,4-tetranitro 1,5 pentanediol, 4,4,6,6,8,8-hexanitro- 1,11-undecanediol, 5,5,5-trinitro 1,2 pentanediol, 5,5- dinitro-l,2-hexanediol, and 2,2-dinitro-l,3-propanediol; nitrazaalkylene diols such as 3-nitraza-1,5-pentanediol, 3,6-dinitraza-1,8-octanediol, and 2-nitraza-1,4-butanediol; and nitrazanitroalkylene diols such as 5-aza,3,3,5 ,7,7-pentanitro-1,9-nonanediol and 6-aza-3,6-dinitro-1,8-octanediol.

Examples of isocyanates useful as starting materials for the preparation of nitropolyurethane propellant binders are lower alkylene diisocyanates such as methylene diisocyanate, ethylene diisocyanate, and 1,3-propane diisocyanate; nitroalkylene diisocyanates such as 3,3-dinitro-1,5-pentane diisocyanate, 3,3,5,7,7-pentanitro-1,9- nonane diisocyanate, 2,2,4,4,-tetranitro-1,5-pentane diisocyanate, and 5,5,5-trinitro-1,2-pentane diisocyanate; nitrazaalkylene diisocyanates such as 3,6-dinitraza-1,8-octane diisocyanate, 3-nitraza-1,5-pentane diisocyanate, and 2-nitraza-1,4-pentane diisocyanate; and nitrazanitroalkylene diisocyanates such as 5-aza-3,3,5,7,7-pentanitro-1,9- nonane diisocyanate, 6-aza-3,6-dinitro-1,8-octane diisocyanate, and 5-aza-3,3,5-trinitro-1,9-nonane diisocyanate.

Mixtures of any of the above-named alcohols and isocyanates can be polymerized to form propellant binders within the scope of this invention.

Illustrative of other solid propellant binders suitable for use in the novel propellants of our invention are those disclosed in U.S. Pat. 2,479,828 and British Pat. 579,057.

Still other types of binders suitable for use in our novel solid propellant compositions are nitrocellulose-plasticizer binders of the type prepared by curing mixtures of finely divided nitrocellulose and suitable plasticizers such as pentaerythritol trinitrate. Binders of this type and their methods of preparation are well known to those skilled in the propellant art.

Still other binder materials useful in the practice of this invention are the low molecular weight isoolefinpolyolefin copolymers disclosed in assignees copending US. Patent Application Ser. No. 202,351, filed June 8, 1962, now Pat. No. 3,399,087.

A finely divided nitrocellulose suitable for use in the preparation of the subject binders is obtained by first dissolving nitrocellulose, preferably prepared from cotton linters, in a solvent such as an ethyl acetate-acetone mixture, an ethyl acetate-ethanol mixture, or nitromethane to form a lacquer. The lacquer is slurried in an aqueous medium containing a suspending agent such as methyl cellulose in combination with an emulsifier such as turkey red oil and an agent to prevent agglomeration such as, for example, sodium chloride as a result of which the nitrocellulose precipitates from the solvent and is recovered as a particulate material having an average particle slze of 10-12 microns and an over-all particle size range of from about 1 to about 35 microns. Finely divided nitrocellulose such as that prepared by the above-described method is known to those skilled in the art as plastisol grade nitrocellulose and will be hereinafter referred to as such. Plastisol grade nitrocellulose is readily available on the open market.

The metal oxide component of our propellants is preferably in finely divided form. The particle size of the metal oxide is not critical, but we have found that the use of the metal oxides which are less than 325 mesh (Tyler) are normally more effective and thus preferred as resonance suppressors.

The propellants of this invention contain, as oxidizers, oxidizing salts such as the chromates, dichromates, permanganates, nitrates, chlorates, and perchlorates of ammonia, hydrazine, guanidine, etc. The selection of the oxidizing salt depends upon the specific burning properties desired in the propellant grain. Mixtures of suitable inorganic oxidizing salts can be used within the scope of this invention.

Various additives may be employed in preparing the preferred polyurethane binders of this invention. For example, plasticizers familiar to those skilled in the art, such as, isodecyl pelargonate; dioctyl azelate; etc. may be utilized. The plasticizer-gives plastic properties to the propellant but does not enter into the polymerization reaction. Also, catalysts for the polyurethane reaction such as triethylamine and other tertiary amines; ferric acetylacetonate and other metal acetylacetonates such as vanadyl acetylacetonate, etc.; boron trifluoride, etc., can be employed if desired. The catalysts can be employed in quantities within the range from mere traces up to amounts equivalent to about one percent by weight of the total propellant composition, and even higher. Normally amounts of from about 0.02 to about 0.10 percent by weight, on a total weight basis, are employed.

The polyurethane polymerization reaction may be carried out either in a suitable solvent or in the absence of a solvent. The solvent may be present in such great excess as to form a solution of the monomers or it may be used in relatively small quantities. Suitable solvents are those in which the various ingredients of the reaction mixture are soluble, such as 4-nitrazapentenoate, dioxane, dimethylphthalate, etc.

Burning rate modifiers and other additives such as antioxidants, wetting agents, anti-foaming agents, etc., can be employed, if desired, in the formulation of our novel propellant s. In this connection, we have discovered that copper chromite and finely divided carbon black, when utilized in small quantities (comprising preferably not greater than 1 percent, of the total propellant weight) are useful for increasing the burning rate of the propellant. We have also found certain well-known Wetting agents, such as lecithin, to be useful processing aids in the preparation of our novel propellants. A wetting agent which we have found to be particularly suitable for our purpose is that known commercially as 6-2684. G-2684 is a mixture of sorbitan monooleate and polyoxyethylene esters of mixed fatty and resin acids, manufactured by Atlas Powder Company of Wilmington, Del. For best results, we have determined that wetting agents should be employed in proportions comprising not more than 1 percent by weight of the total propellant composition and preferably in proportions much lower than this. Various additives other than those specifically mentioned can be employed, in minor amounts, within the scope of our invention. For example, phenyl betanaphthylamine can be utilized in very small quantities as an antioxidant.

In preparing the propellants of this invention, the polyurethane polymerization can be conducted at any temperature, the only effect of temperature variation bemg a corresponding increase or decrease in the rate of reaction. The polymerization readily takes place at room temperature but higher temperatures increase the rate of reaction and are therefore desirable in many cases. As explained above, however, temperatures lower than as well as higher than room temperature can be used for our polymerization reaction.

Because higher temperatures tend to produce shrinkage and internal strains, it is preferable to carry out the cure at temperatures in the range of from about 70 to about 180 F. Within this range the reaction rate is sufficiently rapid for economical production. Yet the temperature is not so high as to produce shrinkage and internal stresses which must be avoided at all costs-especially in the case of large solid propellant motors.

Those skilled in the art will appreciate the fact that heating and cooling steps can be incorporated into our propellant processing procedure to attain optimum operating conditions for producing a given specific propellant. Likewise, various techniques which may serve to optimize the processing procedure or improve the quality of the product, e.g., vacuumizing the mixture during certain phases of the operation, can be employed if desired.

The various processing steps can be carried out with standard equipment well known to those skilled in the art as suitable for the purpose. A mixer which we have found to be particularly effective for mixing our propellant ingredients, however, is that known commercially as the P mixer. The P mixer is manufactured by Baker-Perkins, Inc., of Saginaw, Mich., and it can be equipped with facilities for heating, cooling, and vacuumizing propellant batches during mixing, when such operations are desired.

There are many ways of processing the various ingredients within the scope of this invention in the formulation of propellants therefrom, and these procedures may be readily determined by those skilled in the art, depending on the precise binder, plasticizer, etc., selected and size of the batch to be prepared. In preparing polyurethane propellants, We have found it preferable to add the metal oxide to one or more of the liquid binder components of the system prior to incorporating the oxidizer and other ingredients therein, the principal reason for this being one of safety. Powdered metals, e.g. powdered aluminum is known to be explosive in the presence of oxygen and a hazard is created where aluminum oxide is permitted to contact a dry oxidizing material. Our preferred method of aluminum oxide addition precludes its contact with the dry oxidizer and hence there is substantially no danger of explosion When this procedure is followed. For example, where the polyurethane reactants are diols and diisocyanates and the cross-linkers are polyhydroxy compounds, the diol can be first mixed with the cross-linker and the metal oxide added to the liquid mixture, after which the inorganic oxidizer and the diisocyanate can be stirred or otherwise mixed into the mass. Catalysts and/or other additives can be introduced into the mixture prior to or at the same time as the addition of the diisocyanate or subsequent to this addition. The various additives do not all have to be added at the same stage of processing and, in fact, it has been found preferable in most cases to deviate from this procedure.

One technique which we have found to be quite satisfactory (where the major ingredients and order of addition of these ingredients are as described above) comprises addition of the wetting agent or agents, along with the plasticizer, to the diol, metal oxide, and cross-linker in the mixer; addition of the burning rate modifiers (such as copper chromite and carbon black) during addition of the inorganic oxidizer; and addition of the curing catalyst (such as ferric acetylacetonate) along with addition of the diisocyanate. Modifications of the above methods of introducing the additives, such as, for example, addition of the wetting agents to the diol prior to introduction into the mixer, are varied and many. Likewise, there are many techniques for processing the major components in the preparation of our novel propellants. For example, the diol can first be mixed with the aluminum oxide and then with the inorganic oxidizer, after which the diisocyanate can be added, along with the catalyst and crosslinker.

After the propellant batch has been mixed to substantial uniformity, it is cast, extruded, or compressionformed to the desired shape and cured at a temperature preferably within the range from about to about F. As pointed out above, the propellant mixture can be cast directly into a rocket chamber lined with an inert liner material, and polymerized (cured) therein if this procedure appears to be desirable.

Very small amounts, in the order of about one or two percent by weight (total propellant weight basis), or even less, of the metal oxide are employed for purposes of this invention. It is within the scope of the invention, however, to employ metal oxide in an effective amount up to about 5 percent by weight of the propellant and more preferably in quantities from about 0.2% to about 2% by weight of the propellant. The propellant binder is preferably employed in a proportion within the range from about to about 55 percent and the inorganic oxidizing salt in an amount within the range from about 95 to about 45 percent by weight. The term binder when used herein to de note a polyurethane binder includes not only the diol (or equivalent) and diisocyanate (or equivalent) reaction product but any cross-linker present as well.

The proportions of the ingredients which go to make up the binder, or as it is sometimes called, fuel, can vary through wide ranges, depending on the properties desired in the propellant and the specific reactants employed. Although stoichiometric proportions of hydroxy and isocyanate components can be employed in the preparation of our polyurethane propellants, we have found that a product of improved mechanical properties is obtained if a slight excess of isocyanate groups over hydroxy groups is present in the fuel mixture. Consequently, for best results we have found that there should be from about 100 to about 115 equivalents of isocyanate or isothiocyanate containing monomer in the fuel mixture for every 100 equivalents of hydroxy or thiol containing monomer therein.

There can, of course, be more than one isocyanate compound or equivalent, as well as more than one hydroxy compound or equivalent, in the fuel mixture, in which case the calculation of excess isocyanate over hydroxy groups is based upon the total amounts of all pertinent compounds present. For example, where the cross-linker is a polyhydroxy compound the excess of isocyanate compound (or equivalent) is calculated as an excess over the amount of diol (or its equivalent) plus the amount of cross-linker. The relative proportions of diol and crosslinker can vary through wide ranges so long as a crosslinked structure is obtained in the fuel.

The various additives and minor components of our novel polyurethane propellants (these ingredients other than the urethane and cross-linker reactants) normally comprise a very small percentage of the total propellant weight. Thus, they will usually be present in combined amount not greater than that corresponding to about percent (and preferably about 4 or 5 percent) of the total propellant weight.

The following examples are included for purposes of illustrating the novel process and propellant compositions of our invention. Applicants wish to emphasize that these examples are intended for illustrative purposes only and that they should not be construed as limitative of the scope of the invention to the particular conditions and embodiments set forth therein.

EXAMPLE I This Example describes a particular method of preparing a novel propellant composition according to this invention from the following ingredients:

Ingredient: Weight percent Ammonium perchlorate 81.50 Aluminum oxide 2.00 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.21 Copper chromite 0.50 Tolylene diisocyanate 1.62

The aluminum oxide is stirred into about one-third of the required volume of polypropylene glycol and glycerol monoricinoleate. The mixture is prepared in a stainless steel container. Mixing is continued for about 10 minutes. The aluminum oxide slurry is added to a conventional mixer equipped with facilities for heating, cooling and 12 vacuumizing the propellant mix. The walls of the aluminum oxide slurry container are scraped thoroughly. The container is rinsed with the dioctyl azelate and the rinses are added to the mixture. The remaining polypropylene glycol is added to the mixer.

With the mixer off, the ferric acetylacetonate, phenyl betanaphthylamine and lecithin are added through a 40 mesh screen. The copper chromite is added to the mixture. The mixture is covered and mixed for about 15 minutes under 26-28 inches of vacuum, after which it is stopped and vacuum released. The oxidizer is then added, with the mixer blades in motion. After all of the oxidizer has been added, the mixer is stopped and scraped down. The propellant mass is mixed for 15 minutes at 70 F. under 26 inches vacuum. The mixer is stopped and the vacuum released. The tolylene diisocyanate is added, after which the mass is mixed for 10 minutes at 70 F. and 26 inches of vacuum. The vacuum is then released and the mixture is cast.

The following are other propellant formulations from which propellant grains are prepared according to methods similar to that described in Example I.

EXAMPLE II Ingredient: Weight percent Ammonium perchlorate 82.00

Aluminum oxide 2.00 Copper chromite 1.00 Lecithin 0.20 Phenyl betanaphthylamine 0.20 Polypropylene glycol 8.95 Glycerol monoricinoleate 1.10 Dioctyl azelate 3.00 Ferric acetylacetonate 0.04 Tolylene diisocyanate 1.51

EXAMPLE III Ingredient: Weight percent Ammonium perchlorate 81.50

Silicon dioxide 2.00 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.21 Copper chromite 0.50 Tolylene diisocyanate 1.62

EXAMPLE IV Ingredient: Weight percent Ammonium perchlorate 81.50 Thorium oxide 2.00 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.21 Copper chromite 0.50 Tolylene diisocyanate 1.62

EXAMPLE V Ingredient: Weight percent Ammonium perchlorate 81.50 Zirconium oxide 2.00 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.21 Copper chromite 0.50 Tolylene diisocyanate 1.62

EXAMPLE VI Ingredient: Weight percent Ammonium perchlorate 81.50 Titanium oxide 2.00 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.21 Copper chromite 0.50 Tolylene diisocyanate 1.62

EXAMPLE VII Ingredient: Weight percent Ammonium perchlorate 81.50 Lanthanum oxide 2.00 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.21 Copper chromite 0.50 Tolylene diisocyanate 1.62

EXAMPLE VIII Ingredient: Weight percent Ammonium perchlorate 81.50 Aluminum oxide 2.00 Copper chromite 0.50 Carbon black 0.10 Polypropylene glycol 9.63 Glycerol monoricinoleate 1.18 Dioctyl azelate 3.12 Ferric acetylacetonate 0.04 Phenyl betanaphthylamine 0.20 Lecithin 0.1 1 Tolylene diisocyanate 1.62

EXAMPLE D This Example describes motor firing tests in which, propellant grains with and without resonance suppressor were fired in a test motor to obtain data from which pressuretime curves were plotted. Pressure-time curves are useful in indicating whether or not and to what degree particular propellants are subject to resonant burning.

A 60 lb. batch of propellant was mixed, cast and cured to form an internal-external burning tubular propellant grain. The grain contained no resonance suppressor and was to be used as a control for a series of motor firing tests. Its formulation was as follows:

Ingredient: Weight percent Ammonium perchlorate 81.50 Copper chromite 0.50 Polyurethane fuel 18.00

Weight percent Weight percent aluminum oxide: polyurethane fuel 2.0 26.0 1.0 17.0 0.2 17.8

The four propellant grains described above were conditioned at 75 F. and then each was subjected to a static firing test under identical conditions. During the test pressures within the combustion chamber were recorded. FIGS. 1, 2, 3 and 4 of the accompanying drawings depict pressure-time curves from the firing tests. Turning first to FIG. 1, it will be noted that the pressure-time curve shown there is jagged with many pressure peaks evident. FIG. 1 is the pressure-time curve of the control test, and it is quite obvious from its many sharp pressure peaks of substantial magnitude that the propellant exhibited a high degree of resonance during burning. Contrasted with the rough and jagged nature of FIG. 1 curve are the smooth curves of FIGS. 2, 3 and 4 which show the chamber pressure conditions during firing of the three aluminum oxidecontaining propellants. A comparison of any of the latter three curves with that of FIG. 1 graphically illustrates the effectiveness of small quantities of aluminum oxide in eliminating the problem of resonant burning which has heretofore plagued the solid propellant industry.

Propellant grains similar to those tested, only containing zirconium oxide, silicon oxide, thorium oxide, titanium oxide and lanthanum oxide rather than aluminum oxide as resonance suppressors were made up and subjected to motor firing tests. Here, as in the case of the aluminum oxide-containing grains, the pressure-time curves were smooth, showing an absence of resonant burning during the firing test.

It will be understood that various modifications may be made in this invention without departing from the spirit thereof or the scope of the appended claims.

We claim:

1. A solid propellant composition which comprises a cured intimate mixture of a nonmetallic oxidizing salt, a resin binder selected from the group consisting of polyurethane resin, polyester resin, acrylate resin, polysulfide polymer, nitrocellulose plasticizer, and nitropolyurethane resin, and, as a resonance suppressor, a material selected from the group consisting of aluminum oxide, zirconium oxide, silicon dioxide, thorium oxide, titanium oxide, lanthanum oxide, and mixtures thereof, in an amount effective to suppress resonant burning up to an amount not greater than that equivalent to about 5 percent by weight of the propellant composition, said resin binder being present in an amount between about 5 percent and about 55 percent by weight and the solid non-metallic oxidizing salt being present in an amount between about and about 45 percent by weight, all percentages being given by total propellant weight basis.

2. A solid propellant composition which comprises a cured intimate mixture of a nonmetallic oxidizing salt, a polyurethane resin binder, and, as a resonance suppressor, a material selected from the group consisting of aluminum oxide, zirconium oxide, silicon dioxide, thorium oxide, titanium oxide, lanthanum oxide, and mixtures thereof, in an amount effective to supress resonant burning up to an amount not greater than that equivalent to about 5 percent by weight of the propellant composition, said resin binder being present in an amount between about 5 percent and about 55 percent by weight and the solid nonmetallic oxidizing salt being present in an amount between about 95 and about 45 percent by weight, all percentages being given by total propellant weight basis.

3. A solid propellant composition which comprises a cured intimate mixture of a solid nonmetallic oxidizing salt and a cross-linked resin binder which comprises the reaction product of a compound having, as its sole reacting groups, not less than two active hydrogen groups capable of polymerizing with an isocyanate and a compound having, as its sole reacting groups, not less than two groups capable of undergoing a urethane-type reaction with hydroxy groups and, as a resonance suppressor, a material selected from the group consisting of aluminum oxide, zirconium oxide, silicon dioxide, thorium oxide, titanium oxide, lanthanum oxide and mixtures thereof, the resonance suppressor material being present in an amount effective to suppress resonant burning up to an 15 amount not greater than that equivalent to about five percent by weight of'the propellant composition, said resin binder being present in an amount between about 5 percent and about 55 percent by weight and the solid nonmetallic oxidizing salt being present in an amount between about 95 and about 45 percent by weight, all percentages being given by total propellant weight basis.

4. A solid propellant composition which comprises a cured intimate mixture of a solid nonmetallic oxidizing salt and a cross-linked resin binder which comprises the reaction product of a compound having, as its sole reacting groups, not less than two active hydrogen groups capable of polymerizing with an isocyanate and a stoichiometric excess of a compound having, as its sole reacting groups, not less than two groups capable of undergoing a urethane type-reaction with hydroxy groups; the stoichiometric excess being calculated as an excess over all active hydrogen groups capable of polymerizing with an isocyanate initially present and, as a resonance suppressor, a material selected from the group consisting of aluminum oxide, zirconium oxide, silicon dioxide, thorium oxide, titanium oxide, lanthanum oxide, and mixtures thereof, the resonance suppressor material being present in an amount effective to suppress resonant burning up to an amount not greater than that equivalent to about five percent by weight of the propellant composition, said resin fbinder being present in an amount between about 5 percent and about 55 percent by weight and the solid nonmetallic oxidizing salt being present in an amount between about 95 and about 45 percent by weight, all percentages being given by total propellant weight basis.

5. The solid propellant composition of claim 4 wherein the stoichiometric excess of reactant material containing groups capable of undergoing a urethane-type reaction with hydroxy groups over the reaction material containing active hydrogen groups capable of polymerizing with an isocyanate corresponds to a proportion of from about 100 to about 115 equivalents of the former for every 100 equivalents of the latter.

6. A solid propellant composition which comprises a cured intimate mixture of a solid nonmetallic oxidizing salt and a resin binder which comprises the reaction product of a compound having two active hydrogen groups capable of polymerizing with an isocyanate, selected from the group consisting of:

(a) alkene diols having a chain length of from 2 to 20 carbon atoms, inclusive;

(b) alkane dithiols having a chain length of from 2 to 20 carbon atoms;

(c) alkene diols;

(d) alkene dithiols;

(e) cycloalkylene diols;

(f) cycloalkylene dithiols;

(g) aromatic diols;

(h) aromatic dithiols;

(i) aliphatic ether diols;

(j) aliphatic ether dithiols;

(k) aliphatic amido diols;

(l) aliphatic amido dithiols;

(m) dihydroxy polyesters having a molecular weight of from about 1000 to about 2500; (n) polyalkylene ether glycols having a molecular weight from 400 to about 10,000; polysulfides with glycol end groups; and mixtures thereof; a compound selected from the group consisting of:

(1) alkane diisocyanates;

(2) alkane diisothiocyanates;

(3) alkene diisocyanates;

(4) alkene diisothiocyanates;

(5) alkylidene diisocyanates;

(6) alkylidene diisothiocyanates;

(7) cycloalkylene diisocyanates;

(8) cycloalkylene diisothiocyanates;

(9) cycloalkylidene diisothiocyanates;

(10) cycloalkylidene diisothiocyanates;

( 11) aromatic diisocyanates;

(12) aromatic diisothiocyanates; and mixtures thereof; and, as a cross-linking agent, a compound having as its sole reacting groups, not less than three groups polymerizable with a radical selected from the group consisting of hydroxy, thiol, isocyanate, and isothiocyanate groups; and, as a resonance suppressor, a material selected from the group consisting of finely divided aluminum oxide, finely divided zirconium oxide, finely divided silicon dioxide, finely divided thorium oxide finely divided titanium oxide, finely divided lanthanum oxide, and mixtures thereof, the resonance suppressor material being present in an amount effective to suppress resonant burning up to an amount not greater than that equivalent to about five percent by weight of the propellant composition, said resin binder being present in an amount 'between about 5 percent and about 55 percent by weight and the solid nonmetallic oxidizing salt being present in an amount between about and about 45 percent by weight, all percentages being given by total propellant weight basis.

7. The solid propellant composition of claim 5 wherein the resin binder comprises the reaction product of a stoichiometric excess of the compound selected from the group consisting of:

(1) alkane diisocyanates;

(2) alkane diisothiocyanates;

(3) alkene diisocyanates;

(4) alkene diisothiocyanates;

(5) alkylidene diisocyanates;

( 6) alkylidene diisothiocyanates;

(7) cycloalkylene diisocyanates;

(8) cycloalkylene diisothiocyanates;

(9) cycloalkylidene diisocyanates;

(10) cycloalkylidene diisothiocyanates;

(11) aromatic diisocyanates;

(l2) aromatic diisothiocyanates; and mixtures thereof; the stoichiometric excess being calculated as an excess over the combined equivalents of the compound having two hydrogen groups capable of polymerizing with an isocyanate and the cross-linking agent.

8. The solid propellant composition of claim 6 wherein the resin binder comprises the reaction product of from about to about equivalents of the compound selected from the group consisting of:

(1) alkane diisocyanates;

(2) alkane diisothiocyanates;

(3) alkene diisocyanates;

(4) alkene diisothiocyanates;

(5) alkylidene diisocyanates;

(6) alkylidene diisothiocyanates;

(7) cycloalkylene diisocyanates;

(8) cycloalkylene diisothiocyanates;

('9) cycloalkylidene diisocyanates;

(10) cycloalkylidene diisothiocyanates;

( 11) aromatic diisocyanates;

(12) aromatic diisothiocyanates; and mixtures thereof; for every 100 equivalents of the compound having two active hydrogen groups capable of polymerizing with an isocyanate plus the cross-linking agent.

9. A solid propellant composition which comprises a cured intimate mixture of finely divided aluminum oxide, a solid nonmetallic oxidizing salt, and a resin binder which comprises the reaction product of an aromatic diisocyanate, a polyether having a molecular weight between about 400 and 10,000, and a trihydroxy crosslinker compound; the aluminum oxide being present in an amount effective to suppress resonant burning up to an amount not greater than about five percent by weight, the resin binder being present in an amount between about 5 and about 55 percent by weight, and the solid nonmetallic oxidizing salt being present in an amount between about 95 and about 45 percent by weight, all percentages being given on a total propellant weight basis.

10. The solid propellant composition of claim 9 wherein the aromatic diisocyanate is present in stoichiometric excess, the stoichiometric excess having been calculated as an excess over the combined amounts of polyether and trihydroxy compound initially present.

11. A solid propellant composition which comprises a cured intimate mixture of finely divided aluminum oxide, a solid nonmetallic oxidizing salt, and a resin binder which comprises the reaction product of an aromatic diisocyanate, a polyether having a molecular weight between about 400 and about 10,000 and, as a cross-linker, glycerol monoricinoleate; the aluminum oxide being present in an amount elfective to suppress resonant burning up to an amount not greater than about five percent by weight, the resin binder being present in an amount between about and about 55 percent by weight, the nonmetallic oxidizing salt being present in an amount between 95 and about 45 percent by weight, all percentages being given on a total propellant weight basis, and the aromatic diisocyanate being present in stoichiometric excess, the stoichiometric excess having been calculated as an excess over the combined amounts of polyether and glycerol monoricinoleate initially present.

12. A solid propellant composition which comprises a cured intimate mixture of finely divided aluminum oxide, :1 solid nonmetallic oxidizing salt, and a resin binder which comprises the reaction product of 2,4-tolylene diisocyanate, polypropylene glycol having a molecular weight of from about 2,000 to about 3,000 and glycerol monoricinoleate, the aluminum oxide being present in an amount effective to suppress resonant burning up to an amount not greater than about five percent by weight, the resin binder being present in an amount between about 5 and 55 percent by weight, and the solid nonmetallic oxidizing salt being present in an amount between about 95 and about 45 percent by weight, all percentages being given on a total propellant weight basis.

13. A solid propellant composition which comprises a cured intimate mixture of finely divided aluminum oxide, a solid nonmetallic oxidizing salt, a resin binder which comprises the reaction product of 2,4-tolylene diisocyanate, polypropylene glycol having a molecular weight of from about 2,000 to about 3,000, and glycerol monoricinoleate, the aluminum oxide being present in an amount effective to suppress resonant burning up to an amount not greater than about five percent by weight, the resin binder being present in an amount between about 5 and about 55 percent by weight, the nonmetallic oxidizing salt being present in an amount between about 95 and about 45 percent by weight, all percentages being given on a total propellant Weight basis, and the 2,4-tolylene diisocyanate being present in stoichiometric excess, the stoichiometric excess having been calculated as an excess over the amounts of polypropylene glycol and glycerol monoricinoleate initially present.

14. The solid propellant composition of claim 12 wherein the aluminum oxide is present in an amount not greater than that equivalent to about 2 percent by weight of the propellant composition.

15. The solid propellant composition of claim 12 wherein the nonmetallic oxidizing salt is ammonium perchlorate.

16. The method of preparing a solid propellant composition which comprises intimately dispersing a solid nonmetallic oxidizing salt and, as a resonance suppressor, a material selected from the group consisting of finely divided aluminum oxide, finely divided zirconium oxide, finely divided silicon dioxide, finely divided thorium oxide, finely divided titanium oxide, finely divided lanthanum oxide, and mixtures thereof, in a binder mixture compris- 18 ing a compound having two active hydrogen groups capable of reacting with an isocyanate, selected from the group consisting of:

(a) alkane diols having a chain length of from 2 to 20 carbon atoms, inclusive;

(b) alkane dithiols having a chain length of from 2 to 20 carbon atoms;

(c) alkene diols;

(d) alkene dithiols;

(e) cycloal kylene diols;

(f) cycloalkylene dithiols;

(g) aromatic diols;

(h) aromatic dithiols;

(i) aliphatic ether diols;

(j) aliphatic ether dithiols;

(k) aliphatic amido diols;

(l) aliphatic amido dithiols;

(m) dihydroxy polyesters having a molecular weight from about 1000 to about 2500; (n) polyalkylene ether glycols having a molecular weight from about 400 to about 10,000; (0) polysulfides with glycol end groups; and mixtures thereof; a compound selected from the group consisting of:

(1) alkane diisocyanates;

(2) alkane diisothiocyanates;

(3) alkene diisocyanates;

(4) alkene diisothiocyanates;

(5) alkylidene diisocyanates;

(6) alkylidene diisothiocyanates;

(7) cycloalkylene diisocyanates;

(8) cycloakylene diisothiocyanates;

(9) cycloalkylidene diisocyanates;

(10) cycloalkylidene diisothiocyanates;

( 11) aromatic diisocyanates;

(12) aromatic diisothiocyanates; and mixtures thereof; curing the mixture; the resonant suppressor material being added in an amount efiective to supress resonant burning up to an amount not greater than that equivalent to about 5 percent by weight of the propellant composition.

17. The method of claim 16 wherein the resonance suppressor material is finely divided aluminum oxide.

18. The method of claim 16 wherein the binder mixture is employed in amounts between about 5 and about 55 percent by weight, and the solid nonmetallic oxidizing salt is employed in an amount between about and about 45 percent by weight, all percentages being given on a total propellant weight basis.

19. The method of claim 16 wherein the mixture is cured within the temperature range from about 60 to about 200 F.

References Cited UNITED STATES PATENTS 2,991,167 7/1961 Burton 149-19 3,000,714 9/1961 Bacchelder et al 14919 3,028,271 4/1962 Dixon et a1 149-19 3,073,730 1/1963 Doe et a1. 149-19 2,962,368 11/1960 Guth 149-19 3,017,748 1/ 1962 Burnside 149-19 X OTHER REFERENCES Marsh: Industrial and Engineering, Chemistry, vol. 52, No. 9, September 1960, pp. 768-71.

BENJAMIN R. PADGE'IT, Primary Examiner US. Cl. X.R. 

1. A SOLID PROPELLANT COMPOSITION WHICH COMPRISES A CURED INTIMATE MIXTURE OF A NONMETALLIC OXIDIZING SALT, A RESIN BINDER SELECTED FROM THE GROUP CONSISTING OF POLYURETHANE RESIN, POLYESTER RESIN, ACRYLATE RESIN, POLYSULFIDE POLYMER, NITROCELLULOSE PLASTICIZER, AND NITROPOLYURETHANE RESIN, AND AS A RESONANCE SUPPRESSOR, A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE, ZIRCONIUM OXIDE, SILICON DIOXIDE, THORIUM OXIDE, TITANIUM OXIDE LANTHANUM OXIDE, AND MIXTURES THEREOF, IN AN AMOUNT EFFECTIVE TO SUPPRESS RESONANT BURNING UP TO AN AMOUNT NOT GREATER THAN THAT EQUIVALENT TO ABOUT 5 PERCENT BY WEIGHT OF THE PROPELLANT COMPOSITION, SAID RESIN BINDER BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 5 PERCENT AND ABOUT 55 PERCENT BY WEIGHT AND THE SOLID NON-METALLIC OXIDIZING SALT BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 95 AND ABOUT 45 PERCENT BY WEIGHT, ALL PERCENTAGES BEING GIVEN BY TOTAL PROPELLANT WEIGHT BASIS. 